Color images are customarily obtained in the photographic art by reaction between the oxidation product of a silver halide color developing agent (i.e., oxidized aromatic primary amino developing agent) and a dye-forming compound known as a coupler. The reaction between coupler and oxidized color developing agent results in coupling of the oxidized color developing agent at a reactive site on the coupler, known as the coupling position, and yields a dye. The dyes produced by coupling are indoaniline, azomethine, indamine, or indophenol dyes, depending upon the chemical composition of the coupler and the developing agent. The subtractive process of color image formation is ordinarily employed in multicolored photographic elements and the dyes produced by coupling are usually cyan, magenta or yellow dyes which are formed in or adjacent silver halide emulsion layers sensitive to radiation absorbed by the image dye; i.e., silver halide emulsion layers sensitive to the red, green or blue regions of the spectrum.
The couplers which typically are employed to produce cyan dyes are phenols and naphthols. They yield azomethine dyes upon coupling with oxidized aromatic primary amino color developing agents.
Phenol couplers containing a ureido group in the 2-position are described in U. K. Patent No. 1,011,940 and U.S. Pat. Nos. 3,446,622, 3,996,253, 3,758,308 and 3,880,661. These couplers generally have good light stability. However, many of them yield dyes having absorption maxima (.lambda..sub.max) in the shorter wavelength portion of the red region of the spectrum or have relatively broad spectral absorption curves, or both. Thus, the dyes have undesirable hues for photographic purposes and frequently have significant absorption in the green region of the spectrum. In addition, a number of the dyes fade when contacted with ferrous ion and thus have poor stability in commonly employed processing compositions.
Phenolic cyan-dye-forming couplers which are characterized by a p-cyanophenylureido group in the 2-position of the phenol are highly advantageous. Such couplers are described in Lau, U.S. Pat. No. 4,333,999 issued June 8, 1982. Among the important advantages of this class of couplers are the following:
(1) They have absorption maxima (.lambda..sub.max) in the longer wavelength portion of the red region of the visible spectrum (generally above 750 nm) and thus yield dyes of desirable hue for photographic images.
(2) They yield dyes having relatively narrow spectral absorption curves and little absorption in the green region of the spectrum and thus yield sharp cutting dyes of relatively pure hue.
(3) They yield dyes which have excellent stability toward reduction by ferrous ion and, accordingly, can be used in processes employing bleach-fix baths containing ferrous ions without a significant reduction in cyan dye density.
(4) They yield dyes which are stable to heat and light.
The phenolic cyan-dye-forming couplers of U.S. Pat. No. 4,333,999 can be prepared by a method of synthesis involving high pressure catalytic hydrogenation of a 2-(p-cyanophenylureido)-5-nitrolphenol, utilizing hydrogen gas and a transition metal catalyst in specially designed high pressure equipment, to form the corresponding 2-(p-cyanophenylureido)-5-aminophenol and reaction of the 2-(p-cyanophenylureido)-5-aminophenol with an acylating agent to form the phenolic cyan-dye-forming coupler. However, this method of synthesis requires the use of flammable hydrogen gas and specialized high pressure equipment. Moreover, impurities can be generated as a result of side reactions and thereby provide an undesirably low yield of coupler.
It is toward the objective of providing an improved method of synthesis which gives good yields and does not require the use of flammable hydrogen gas nor the use of specialized high pressure equipment that the present invention is directed.